1. Field of the Invention
The present invention relates to an image sensing apparatus in which an image signal processing system of a plurality of systems is provided with a clamp circuit for use in a video camera or the like, particularly, a clamp circuit having a feedback loop and an ordinary clamp circuit.
2. Description of the Prior Art
For example, in an image sensing apparatus such as a video camera, the light from a subject is incident onto an image sensor through a lens system and is photoelectrically converted to image information by the image sensor. The information regarding three primary colors or R(red), G(green) and B(blue) obtained from the image information of the subject is supplied to a chrominance signal processor.
In order to accurately perform the wavefor processes for an image signal, such as addition of pedestal components, clipping, gamma correction, etc. in the chrominance signal processor, a clamping operation to constantly fix the black level of the image signal to a reference level is executed. A feedback clamp circuit is generally used to carry out this clamping operation.
A feedback clamp circuit which is used so far will be described with reference to FIG. 1. In this drawing, a reference symbol A denotes an input terminal. An image signal applied to the input terminal A is input to an image signal processor 11 and is subjected to image signal processes such as a gain control and the like by the processor 11, thereafter the image signal is output from an output terminal B to a signal processor at the post stage. A gate pulse is applied to a gate switch 13 for the optical black interval. When the gate switch 13 is closed, a potential at the point B is held by a capacitor C.sub.2. In this state, the output of the processor 11 is input to the inverting input terminal of a comparator 14 and is compared with a reference voltage input to the non-inverting input terminal, so that the deviation voltage is held by a capacitor C.sub.1 . A clamp pulse is applied to a gate switch 15 for the horizontal blanking interval to connect the gate switch 15, so that the voltage held in the capacitor C.sub.1 is fed back to the point A and is clamped, thereby fixing a potential at the point B to the reference level.
However, this feedback clamp circuit serves to hold the input voltage into the capacitor and the time constant circuit consisting of R and C is equivalently constituted by the resistances which those circuit elements have. Therefore, there is a drawback such that even if a power switch of the image sensing apparatus having such a feedback clamp circuit is turned on, its leading time is slow when compared with an ordinary pulse clamp circuit.
On the other hand, in an image sensing apparatus with an automatic diaphragm which is equipped with an automatic light control system (ALC), a signal processor to perform the automatic light control is connected to the post stage of the foregoing feedback clamp circuit. However, due to a slow leading time of the feedback clamp circuit, the operation of the automatic light control system is also obviously delayed.
Consequently, it is impossible to immediately photograph after the power switch of the image sensing apparatus is turned on. Thus, even in the case where the automatic diaphragm mechanism is utilized to provide an easy operating function for the image sensing apparatus, its use method is restricted. In addition, there are cases where photographic opportunities will have been lost.
Further, there is a drawback that the number of pins increases in making the feedback clamp circuit as an IC for reduction in size of the image sensing apparatus.